Computing devices, such as notebook computers, personal data assistants (PDAs), mobile handsets and the like, all have user interface devices. One class of user interface device that has become more common is based on capacitive touch-sensor technology utilizing touch-sensitive capacitors. Touch-sensitive capacitors may be used to implement touch-sensor pads, such as the familiar mouse pad in notebook computers, non-mechanical slider controls (e.g., a volume control) and non-mechanical push-button controls.
FIG. 1A illustrates a typical touch-sensor pad 100. The touch-sensor pad 100 includes a sensing surface 101 on which a conductive object may be used to position a cursor in the x- and y-axes, or to select an item on a display. Touch-sensor pad 100 may also include two buttons, left and right buttons 102 and 103, respectively, which may operate as touch-sensitive switches.
FIG. 1B illustrates a conventional linear touch-sensor slider (“slider”) that might be used as a linear control such as a volume control, for example. The slider 110 includes a number of conductive sensor elements 111 separated by insulating gaps 112, where each sensor element is an electrode of a capacitor. Typically, a dielectric material (not shown) is overlaid on top of the sensor elements to prevent any direct electrical conduction between the sensor elements and/or a conductive object when the conductive object is placed on the slider. When a conductive object contacts or comes in proximity to one of the sensor elements, a capacitance associated with the sensor element (or with an adjacent pair of sensor elements) is changed. The change in capacitance can be detected and sent as a signal to a processing device. As a finger or other conductive object moves across the slider, the changing capacitance of each sensor element is detected to pinpoint the location and motion of the conductive object. This same principle (i.e., detecting capacitance changes) can also be used to implement touch sensor buttons (e.g., on-off controls).
FIG. 2A illustrates one form of a touch sensitive capacitor 300. In its basic form, the touch sensitive capacitor 300 includes a pair of adjacent plates 301 and 302. There is a small edge-to-edge (fringing) capacitance Cf between the plates. When a conductive object 303 (e.g., a finger) is placed in proximity to the two plates 301 and 302, there is a capacitance between the conductive object and each of the plates. If the capacitance between the conductive object and each plate is defined as 2*CS, then the total capacitance between the plates due to the presence of the conductive object is CS (the series combination of the two separate capacitances). This capacitance adds in parallel to the fringing capacitance Cf between the plates 301 and 302, resulting in a change in total capacitance equal to CS.
FIG. 2B illustrates another form of a touch sensitive capacitor 307 where two parallel plates 305 are separated by a dielectric layer 308 and one of the plates is grounded. Typically, the ungrounded plate is covered by a second dielectric layer 304. The parallel plate capacitance between the two plates 305 is denoted by CPP. When the conductive object 303 approaches or contacts dielectric layer 304, a capacitance CS is created between the conductive object and the ungrounded plate. As a result, the total capacitance from the ungrounded plate to ground is given by the sum of the capacitances CPP+CS (the conductive object need not be actually grounded for the touch sensitive capacitor to operate; a human finger, for example, is connected to a person's body capacitance, which can act as a virtual ground). Detecting a touch is then a matter of measuring the change in capacitance from CPP to (CPP+CS). In a typical touch sensitive capacitor, CS may range from approximately 10 to 30 picofarads (pF), although other ranges may be used. While the conductive object illustrated here is a finger, any conductive object may be used (e.g., a stylus).
A variety of different circuits have been developed that can be used to detect and/or measure the capacitance and/or capacitance changes of touch sensitive capacitors. These circuits include capacitance sensing relaxation oscillators, resistor-capacitor phase shift circuits, resistor-capacitor time constant circuits, capacitive voltage dividers and capacitive charge transfer circuits.
FIG. 3A illustrates a conventional charge transfer circuit. In FIG. 3A, CX is the touch-sensitive capacitance being sensed and CINT is a summing (integration) capacitor. The operation of the circuit is relatively simple. At the start of the measurement cycle, any charge on CINT is discharged by momentarily turning on switch SW3. Switches SW1 and SW2 are operated in a non-overlapping way during the rest of the measurement cycle. First, SW1 is turned on, charging CX to the supply voltage VDD. The charge on CX is given by:QX=CXVDD Next, SW1 is turned off and SW2 is turned on, causing the charge on CX to be distributed between CX and CINT, which lowers the voltage on CX and raises the voltage on CINT. This operation is repeated multiple times, causing the output voltage to increase asymptotically toward VDD. The equation for the output voltage on the nth iteration is given by:
            V      OUT        ⁢                  ⁢          (      n      )        =                    V        OUT            ⁢                          ⁢              (                  n          -          1                )              +          [                        V          DD                -                              V            OUT                    ⁢                                          ⁢                      (                          n              -              1                        )                    ⁢                                          ⁢                                    C              X                                      C              INT                                          ]      This recursive equation has an exponential solution given by:
      V    OUT    =            V      DD        (          1      -              ⅇ                  -                                    nC              X                                      C              INT                                            )  which is illustrated in FIG. 3B. The value of CX can be determined by counting the number of iterations that are needed for VOUT to reach a specified threshold voltage VTH:
  n  =            -                        C          INT                          C          X                      ⁢                  ⁢    ln    ⁢                  ⁢          (              1        -                              V            TH                                V            DD                              )      The value of CX is then given by:
      C    X    =            -                        C          INT                n              ⁢                  ⁢    ln    ⁢                  ⁢          (              1        -                              V            TH                                V            DD                              )      Conversely, the output voltage can be measure after a specified number of iterations (n=N), in which case the value of CX is given by:
      C    X    =            -                        C          INT                N              ⁢                  ⁢    ln    ⁢                  ⁢          (              1        -                              V            OUT                                V            DD                              )      
In both cases, the value of CX is a nonlinear function of n and VOUT. As a result, either extra circuitry is required to linearize the output or lookup tables are needed to map the nonlinear function to linear values. Either approach increases the complexity and cost of the sensor circuitry.